The present invention relates to a multilayer zinc oxide (ZnO), varistor and a composition for use in the multilayer ZnO varistor, in particular, to a composition containing at least 90 mole % of ZnO and at most 10 mole % of additives for the multilayer ZnO varistor with variable breakdown voltages.
Generally speaking, sudden surges of voltage or electric current inevitably occurs in electric or signal circuits. The source of these surges is voltage transients mainly resulting from lightening or, starting up and on/off switching operations of generators or motors. Voltage surges and disturbances can damage electric components of the electric circuits and even cause fire. Varistors, also known as surge absorbers, are normally used to protect electric circuits and electric components against spurious voltage surges and voltage transients.
Varistors are resistors with resistance varying with voltage with a nonlinear coefficient. Varistors have high resistance and are good resistors when loaded with voltage below the critical voltage. However, when voltages are higher than the critical voltage, the resistance of the varistor sharply decreases and the electric current through the varistor will greatly increase. That is, varistors possess the ability to adsorb surges, reduce overload voltage to a safe level and prevent electric components from being damaged by surges. Hence, varistors are called "surge absorbers".
The most important electric characteristic of varistors is the breakdown characteristics which can be represented in accordance with the relationship: EQU I=KV.sup..alpha.
wherein I represents the electric current through the element, K is a constant, V represents the voltage applied across the element and .alpha. is a nonlinear coefficient.
The value of V selected to give a 1 mA current through the element is called the "breakdown voltage", V.sub.1mA. The greater the value of .alpha., the more significant the influence of the voltage on the electric current. In other words, the greater the value of .alpha., the better the voltage control characteristic, and the stronger the protecting ability of the varistor will be for the electric circuit.
Zinc oxide varistors, having an extremely high value of nonlinear coefficient and a significantly excellent surge absorbing capability, are widely applied as surge absorbing elements, arresters, and voltage stabilizer elements, etc. To make compact communication equipment, the trend is to make electric components which are light weight, thin short in length, small in size, having low power consumption, and operating at low voltages. Circuit protecting devices, such as varistors, naturally should also meet the above requirements.
The breakdown voltage of zinc oxide varistors is represented by the equation of EQU V.sub.1mA =V.sub.g .times.D/d
wherein D represents the thickness of the varistor layer between two parallel internal electrodes in a varistor, d represents the size of a parallel grain and V.sub.g represents the breakdown voltage per grain boundary. The V.sub.g value of ZnO varistors is found experimentally to be about 3-4 V which is not influenced by changes in the compositions of additives or manufacture temperatures. Hence, in order to produce zinc oxide varistors with low breakdown voltage, the control of the parameters D and d are important. The grain sizes of the varistor can be controlled by varying the composition and the sintering temperature. For varistors used at low voltages, grain growth promoters such as TiO.sub.2 or seeds are added into the varistor composition to promote the grain growth. However, the addition of the grain growth promoters would result in abnormal grain growth. The distribution of grain sizes is difficult to control and the surge withstanding capabilities of the varistors is reduced. Alternatively, the growth of average grain size can be achieved by increasing the sintering temperature. However, the sintering temperature has an upper limit of about 1,400.degree. C., above which zinc oxide and additives will be evaporated and thereby, the characteristics of the varistor are lost. Hence, the lower limit of the breakdown voltage is also influenced and therefore limited.
For varistors used at low voltages, control of the parameter D can be achieved by reducing the varistor thickness by using either the thin foil method, sandwich method, thick film method or multilayer method. The thin foil method utilizes conventional plate-pressing machines to produce the a varistor thickness of about 0.3 mm which is the lowest limit in this method. However, the precision of thin foil type varistors is difficult to control and the quality thereof is poor. The sandwich method relates to addition of additives into zinc oxide single crystal chips and sintered zinc oxide poly-crystal ceramic chips and then heating at high temperatures to produce sandwich type varistors. The additives will diffuse into okay ceramic chip at the high temperature. The breakdown voltage of the produced varistor is quite low; for instance, about 3 V, but the surge withstanding capability thereof is poor for practical use. The thick film method comprises the steps of forming a slurry of zinc oxide, additives and glass; screen printing heat resistant Pt or Pd conductive gels onto alumina substrate; applying a coating of the slurry of about 100-200 .mu.m thickness thereon; co-firing at high temperature; printing silver gel on the combination obtained above and baking to obtain thick film type varistors. Theoretically, the nonlinear coefficient .alpha. of thick film varistors is only about one half that of ordinary varistors. That means, at low breakdown voltages, it has a low value of nonlinear coefficient .alpha. and needs a substrate. Further, because of the comparatively poor compactness, thick film type varistors have poor surge withstanding capability.
Recently, in U.S. Pat. No. 4,290,041, Utsumi et al. utilized a concept in manufacturing multilayer capacitors to produce multilayer zinc oxide varistors with variable breakdown voltages. The Utsumi's method comprises the steps of tape casting green sheet, printing internal electrodes, laminating, cutting, sintering and applying external electrodes. Pb--B--Zn--Si (Borosilicate-lead-zinc) glass is used to substitute conventional component Bi.sub.2 O.sub.3 in varistor compositions. The characteristics of varistors are originated in the interface of zinc oxide and glass. In order to retard the degradation, an adequate amount of glass is added to manufacture zinc oxide disk-type varistors. However, the addition of glass would reduce the surge withstanding capabilities of the varistors. For this reason, glass is normally not added for disk type varistors in commercial production processes. The addition of Pb--Zn--B--Si glass to substitute Bi.sub.2 O.sub.3 in varistor compositions in Utsumi's patent finally results in poor surge withstanding capability.
In addition to breakdown voltage V.sub.1mA and nonlinear coefficient .alpha., degradation is also an important factor to be considered in practical use. When a varistor is used in an electric circuit, there will be some leak current passing through the varistor. Due to Joule's effect, the temperature of the varistor will increase, and the leakage current will also increase with the increase of the temperature. Furthermore, the value of V.sub.1mA will reduce and the varistors start to degrade. Thus, it is effective to make use of leakage current testing methods to measure the life and reliability of the zinc oxide varistors.